1. Technical Field
The present disclosure relates to a mobile station and a reception quality measurement method.
2. Description of the Related Art
In recent years, in cellular mobile communication systems, it has become popular to provide information in a multimedia form such that not only audio data but also still image data, moving image data, or the like with a large data size is transmitted. In LTE-Advanced (Long Term Evolution Advanced), an active investigation has been made to achieve a high transmission rate using a wireless broadband, a Multiple-Input Multiple-Output (MIMO) transmission technique, and an interference control technique.
Furthermore, in LTE-Advanced, it is under consideration to provide a small cell, which is a base station with low transmission power (also referred to as e Node B (eNB)), to achieve a high transmission rate at a hot spot. It is under consideration that a carrier frequency is assigned to the small cell such that the assigned frequency is different from that used in a macro cell, which is a base station with high transmission power, (see, for example, 3GPP TR 36.872 V12.0.0 (2013-09), Small Cell Enhancements for E-UTRA and E-UTRAN Physical layer Aspects).
It is also under consideration to allow a mobile station (which is also called user equipment (UE) or a terminal) to by itself connect to a small cell. It is also under consideration to use carrier aggregation, in which a plurality of component carriers are used, to allow a mobile station to connect to both a macro cell and a small cell. Furthermore, it is also under consideration to employ dual connectivity to allow a mobile station to connect to a Master eNB (MeNB) and a Secondary eNB (SeNB) (see, for example, 3GPP TR 36.842 V1.0.0 (2013-11), Study on Small Cell Enhancements for E-UTRA and E-UTRAN Higher layer Aspects). In the Dual Connectivity, a cell that manages mobility of mobile stations is called MeNB. Other than MeNB, a cell that assigns a resource to a mobile station is called SeNB. A mobile station is allowed to use both a resource of MeNB and that of SeNB.
In a case where a mobile station connects by itself to a small cell, the mobile station is likely to move to the small cell in response to receiving a handover command from another cell. In a case where the carrier aggregation is used, a small cell is likely to be set as a Secondary Cell (SCell). In the case of the Dual connectivity, a small cell is likely to be set as SeNB. In any case, before a mobile station makes a connection to a cell, the mobile station needs to identify the cell, achieve synchronization with the cell, and measure reception quality between the cell and the mobile station.
Conventionally, the cell synchronization is achieved via Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS) transmitted at intervals of 5 msec. Thereafter, a cell ID is identified and then Radio Resource Management (RRM) is performed using reception power of Cell specific Reference Signal (CRS) and reception power of the whole band.
RRM is used in measurement for mobility management such as a cell selection or the like. In RRM, Reference Signal Reception Power (RSRP) or Reference Signal Reception Quality (RSRQ) is measured. In a case where RSRP of an adjacent cell satisfies a predetermined criterion, for example, in a case where RSRP of the adjacent cell is higher by 3 dB than that of a current cell, a mobile station makes a report of a cell ID and RSRP of this adjacent cell. Here in a case where information on the cell ID of the adjacent cell has been informed, the mobile station is capable of detecting the cell using the cell ID.
RSRP is average reception power of CRS, and RSRQ is given by N*RSRP/RSSI (see, for example, 3GPP TS 36.214 V11.0.0 (2012-9), Physical layer; Measurements) where N is the number of resource blocks (RBs) in a band in which Received Signal Strength Indicator (RSSI) is measured, and RSSI is the average reception power in an OFDM symbol. In a case where no instruction is given from a higher layer, RSSI is measured in an OFDM (Orthogonal Frequency Division Mutiplexing) symbol in which CRS is mapped. On the other hand, in a case where a subframe in which RSSI is to be measured is specified from the higher layer, RSSI is measured in all OFDM symbols in the specified subframe. RSRP corresponds to reception power of a certain cell and RSSI corresponds to reception power of a whole band, and thus RSRQ indicates the ratio of the reception power of the certain cell to the reception power of the whole band including interference. RSRQ is a parameter including an amount of interference varying depending on the band, and thus RSRQ is used in comparison (interband comparison) in terms of reception quality of a cell between different bands.
As described above, in the cell selection based on the RSRQ criterion, a cell is selected by comparing reception quality of cells between different bands. In this process, if traffic per band is large, there are many cells that may cause interference, and thus RSSI has a large value and RSRQ has a small value. On the other hand, if traffic per band is small, there are a small number of cells that may cause interference, and thus RSSI has a small value and RSRQ has a large value. Therefore, in the cell selection based on the RSRQ criterion, the difference in traffic between bands is taken into account, which results in an increase in the probability that a cell with a band having a low traffic is selected. When a cell with a band having a low traffic is selected, less interference is achieved and it is possible to use much resource of the cell, and thus it is possible to advantageously achieve an increase in user throughput.